1. Field of the Invention
This invention relates to a facsimile apparatus and, more particularly, to a facsimile apparatus connected on the ISDN side in a communication network in which the ISDN is connected to a telephone network.
2. Description of the Prior Art
An ISDN is a communication network which provides a wire variety of services, such as telephone, data and facsimile communication services, wherein information such as voice and image data is communicated in the form of a 64 Kbps digital signal by an information channel referred to as a B channel. Signal-converted information for link control, call control, etc., used in communicating with a network is transferred over a 16 Kbps D channel.
Though a public switched telephone network (PSTN) is presently in wide use as a communication network, there are plans to replace with with the ISDN. In the transitional period during which the replacement will be made, both the ISDN and PSTN will coexist for an extended period of time and the two networks will be connected to each other.
In G3 facsimile modulation/demodulation, a procedure signal employs a CCITT V.21 300 bps modem, a picture signal employs a CCITT v.299 600 bps modem, and the bandwidth used is 300 Hz-3.3 KHz. In mutual communication with a G3 facsimile in a network connection between the ISDN and PSTN, an ISDN facsimile converts the transmission analog signal of both modems into a digital signal by CCITT G.711 u-law or A-law coding and then transmits the analog signal, and converts a received digital signal into an analog signal by the above-mentioned coding. The transmitted and received digital signals are transferred to or from a telephone network through the B channel of the ISDN.
An ISDN bearer service, which is defined by CCITT advisory I.211, is classified into voice, 3.1 KHz audio and 64 KHz unlimited digital. Voice refers to a telephone signal, 3.1 KHz audio to a 3.1 KHz band data signal, such as in a G3 facsimile, and 64 KHz unlimited digital to a digital signal such as in a G4 facsimile. Thus, in an ISDN, the information which flows through the network is recognized as being a telephone voice or facsimile image data.
In an ISDN terminal, a terminating procedure of the kind shown in FIG. 3 is executed. By means of a SET-UP message, the network indicates to a called terminal the classification of the transfer capability, namely the voice signal, 3.1 KHz audio signal or 64 KHz unlimited digital signal. The called terminal checks the abovementioned transfer capability and decides whether a response is allowed.
In the telephone network, on the other hand, an end-to-end bus is connected in accordance with a telephone number requested by the calling terminal, and whether the information which flows through the network is a voice signal or facsimile image signal cannot be identified. Accordingly, in a case where a call is placed from a telephone network to the ISDN in a network connection between the telephone network and the ISDN, it is no longer possible to distinguish between a voice and 3.1 KHz audio. This means that an ISDN facsimile will be incapable of recognizing whether a calling terminal is a telephone or a facsimile.